Vehicles and various kinds of industrial machines are employing a large number of surface-hardened parts for the purpose of enhancing the fatigue strength. Typical surface hardening process methods include, for example, carburizing, nitriding and induction hardening.
Unlike the other methods, the nitriding process is performed at a temperature lower than a transformation point of the steel, which makes it possible to reduce the thermal treatment distortion.
Further, the nitriding process can form the effective hardened case (hardened layer) having a depth of 100 μm or more within several hours, which makes it possible to enhance the fatigue strength.
In order to obtain steel parts exhibiting further improved fatigue strength, it is necessary to increase the depth of the effective hardened case. There is proposed a steel having an appropriate amount of alloys added therein to form nitrides, thereby obtaining the effective hardened case having predetermined hardness and depth (for example, Patent Documents 1 and 2).
Patent Document 2 discloses a steel for nitriding including: C: 0.35 weight % to 0.65 weight %, Si: 0.35 weight % to 2.00 weight %, Mn: 0.80 weight % to 2.50 weight %, Cr: 0.20 weight % or less, and Al: 0.035 weight % or less with a balance including Fe and inevitable impurities.
Patent Documents 3 to 7 propose a steel exhibiting improved workability and nitriding property by controlling a microstructure.
For example, Patent Document 5 discloses a steel for nitriding exhibiting excellent cold forgeability, which includes: by weight %, C: 0.01% to 0.15%, Si: 0.01% to 1.00%, Mn: 0.1% to 1.5%, Cr: 0.1% to 2.0%, Al: over 0.10% to 1.00%, V: 0.05% to 0.40%, and Mo: 0.10% to 1.00% with a balance including iron and inevitable impurities, in which the hardness at the core part after the hot rolling or after the hot forging is HV of 200 or less, and the upper limit compression ratio for the cold forging thereafter is 65% or more.
Patent Document 6 discloses a material for nitriding parts exhibiting excellent broaching workability, which includes: by mass %, C: 0.10% to 0.40%, Si: 0.50% or less, Mn: 0.30% to less than 1.50%, Cr: 0.30% to 2.00%, and Al: 0.02% to 0.50% with a balance including Fe and inevitable impurity elements, and the material has a bainite structure having hardness of HV210 or more.
Patent Document 7 discloses a crankshaft including, by mass %, C: 0.10% to 0.30%, Si: 0.05% to 0.3%, Mn: 0.5% to 1.5%, Mo: 0.8% to 2.0%, Cr: 0.1% to 1.0%, and V: 0.1% to 0.5% with a balance including Fe and inevitable impurities, in which: a percentage of bainite is 80% or more, the bainite being obtained in a manner such that a steel test piece satisfying 2.3%≦C+Mo+5V≦3.7%, 2.0%≦Mn+Cr+Mo≦3.0%, and 2.7%≦2.16Cr+Mo+2.54V≦4.0% and taken from a core part not receiving any effect of a nitriding process is austenited at 1200° C. for one hour, and then cooled to a room temperature at a cooling rate of 0.5° C./sec during a time when temperatures change from 900° C. to 300° C.; the Vickers hardness of the crankshaft measured in cross section is in the range of 260 HV to 330 HV; the surface hardness of a nitrided layer of a pin part and a journal part is 650 HV or more; the depth of the nitrided layer formed is 0.3 mm or more; and hardness at the core part is 340 HV or more.
Patent Document 8 discloses a steel for nitrocarburizing including, by mass %, C≦0.15%, Si≦0.5, Mn≦2.5%, Ti: 0.03% to 0.35%, and Mo: 0.03% to 0.8%. The steel has a structure in which the area percentage of bainite after nitrocarburizing is 50% or more, and fine precipitates having a grain diameter of less than 10 nm disperse in a bainite phase, and occupy 90% or more of the total precipitates.